Calculating a Pregnancy Due Date: Why It Is an Estimate

Your pregnancy due date is a useful reference point for appointments and planning. It is an estimate of when your baby might arrive, not a promise about one particular day. Understanding how the date is worked out can make conflicting calendar or app results less confusing.

Where the first estimate comes from

A standard calculation counts 280 days, or 40 weeks, from the first day of your last menstrual period. It assumes a regular cycle of about 28 days. Ovulation timing and cycle length can vary, so this starting calculation does not fit every pregnancy equally well.

If your periods are irregular, or you cannot remember the date, tell your maternity team. Avoid choosing a date simply because a form requires one. You can explain that it is uncertain and ask how to complete the form.

For pregnancies following IVF, the clinic uses treatment information, including the embryo’s age and transfer date. Ask the clinic for the due date to carry into your maternity records.

Why a scan can change the date

An ultrasound in the first trimester can help establish or confirm pregnancy dating. Your clinician compares the available information and records an agreed estimate. Once that date is established, later changes are usually uncommon and should be explained to you.

In NHS care, the dating scan is usually offered around 10 to 14 weeks. The invitation should tell you how to prepare. Ask the service if the instructions are unclear rather than assuming every scan appointment has the same requirements.

A change in the estimate may leave you wondering which week you are now in. Our guide to pregnancy weeks and months explains the counting system used in maternity notes.

Use one agreed date for planning

Later growth measurements do not normally replace a date established by an accurate early scan. A baby measuring smaller or larger needs interpretation in the context of growth, rather than an automatic calendar adjustment.

If the date changes, ask which information prompted the change and whether your upcoming appointments need adjusting. Check that your notes and booking system show the same date. The repeat ultrasound guide can help you separate a scan for dating from one arranged to monitor a particular finding.

Keep the date your maternity team is using in a clear place, alongside any appointment letters. If an app shows a different estimate, check its settings and ask your clinician which date to follow. Recalculating through several websites can produce more questions without adding useful information.

Useful questions include: “Is this date provisional?” and “Which date should I use when arranging my next appointment?” Add them to your notes for the first prenatal appointment.

Leave room around the date

When talking with family, it may feel easier to give an approximate month than an exact date. For practical arrangements, consider who could help if plans need to change, rather than asking everyone to hold one particular day.

Use clinical advice for the timing of tests and care. Use flexibility for social plans, visitors, and the expectations other people place on your calendar.